Stress – not as black as it's painted

How stress influences memory

by
Meike Drießen

November 3, 2014

Retrieving memories under stress does not work well. However, stress can also help to consolidate new memories.

Stress has a bad reputation. It does not feel good, it makes you restless and it blocks your memory: why else should the crucial answer slip your mind in the oral exam which you can normally give in your sleep? "The bad reputation stress has is partly justified; but there's another side to it, too," says Prof Dr Oliver T. Wolf, neuroscientist at the RUB Faculty of Psychology.

The experience in an exam situation is very real: we find it difficult to retrieve memorised information under acute pressure. Several years ago, Oliver Wolf, together with a team of colleagues, demonstrated that stress can have a positive effect on memory. If stress occurs immediately before or after learning, the consolidation of the brain is optimised, i.e. the long-term storage of information is improved. The decisive factor is the stress hormone cortisol – the effect was observed in studies even if the participants took it in form of a tablet immediately before or after learning.

In order to examine the correlation of stress and memory, researchers have been focusing on the stress phase itself: how does learning under stress work? In other words: what are the things that people remember from a stressful episode?

For a study conducted within the framework of the collaborative research centre 874 "Integration and Representation of Sensory Processes", 60 test subjects were subdivided into two groups. Both groups took part in a simulated job interview. One group appeared in front of a two-man panel, dressed in white coats, wearing dispassionate expressions and acting in a non-committed manner (fig. 1). The participants were recorded on tape. The other group met the same panel; however, they did not wear the white coats, were friendly and attentive, smiling and nodding. The interview was not taped. During the test subject's eight-minute free speech, the panel in both cases toyed with various items, such as a pencil sharpener or a glass of water. Other items, e.g. a hole puncher, were sitting on the table, but were not handled. The researchers determined the stress level at various points before and after the stress situation resp. control situation by analysing the cortisol level in saliva.

On the following day, the researchers showed the test subject photos of the items that had been on the table during the interview, those which the panel had handled and those which had not been present in the situation. The participants had to state if they had seen the respective items on the day of the interview or not. "It turned out that the people who had been stressed remembered more items than the relaxed participants," as Oliver Wolf sums up the test results. "In particular the items that the members of the panel had handled have stuck in their memory." This outcome suggested that, when it comes to forming memories, an item had to be closely linked to the stressor. "The evolutionary advantage might have been as follows: emotionally important items are more relevant in stressful situations than neutral ones, which is why they get stored more efficiently," believes Oliver T. Wolf (fig. 2). These findings are also indicative of mechanisms that play a role in post-traumatic stress disorder. In case of PTSD, people who have experienced a life-threatening situation are regularly haunted by memories and nightmares that are linked to the event in question.

Another important aspect for clinical questions is the work undertaken on behalf of the DFG Research Unit 1581, where Oliver Wolf is a contributor. The unit studies another phenomenon related to stress and memory: extinction learning, i.e. "deleting" of memory contents. It is not deleting in the true sense of the word, because memory contents cannot simply vanish; rather, a second memory track is superimposed. The effect is similar, though: contents that was once learned – in clinical terms for example excessive anxiety of something – gets unlearned. Here, too, the researchers investigated the role of stress: if it inhibits the retrieval of memory contents, then it might also inhibit the retrieval of learned fear.

Forty men took part in the relevant experiment(fig. 3). In the first step, all test participants underwent a learning phase: an electrode, through which electrical impulses could be transmitted, was attached to their shins. Together with the participants, the researchers determined a current intensity that was unpleasant, but not painful. On the monitor, the participants were shown the picture of an office with a desk lamp that lit up in different colours. Red and yellow light was followed by electric shocks being applied to two-thirds of the participants. The blue light was not followed by any electric shocks. Analyses of skin conductance that give indication of the level of emotional excitement demonstrated that the participants learned to anticipate the shocks more and more strongly, i.e. they feared them.

Subsequently, the researchers showed the participants the picture of another office, which held the same lamp; the lamp thus appeared in a different context. This time, the yellow and blue light was not accompanied by electric shocks; red light did not occur at all. Thus, the researchers "deleted" the learned information that yellow light was likely to be followed by an unpleasant stimulus.

On the following day, one half of the participants were put under stress: they had to submerge their hands into ice water for three minutes while being videotaped. The other half of the group submerged their hands into tepid water without being videotaped, i.e. they were not stressed. Twenty minutes later, the researchers tested how successful the extinction of the learned fear of electric shocks had been. They showed both monitors from the previous day to both groups. This time, the participants did not receive any electric shocks, no matter which colour the light of the lamp was. "The participants who had submerged their hands into ice water feared the unpleasant stimulus significantly less strongly than those who had not undergone a stressful situation," says Oliver T. Wolf. That was true for both office contexts, i.e. the picture used in the learning situation and the one used in the extinction situation.

"These results are relevant for the treatment of anxiety patients," says Oliver T. Wolf. "In those patients, anxiety that they have unlearned in therapy will still frequently return in their everyday life. This means fear depends on the context." The fact that in stressed patients fear returns with less force on viewing of the office context used in the learning situation gives rise to hope that stress hormones may help unlearn fear regardless of context.

The study results correspond with the research group's older findings, which showed that stress blocks, in particular, the retrieval of emotional material. "In such cases, stress appears to have a protective function," points out Wolf.

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